With U.S. domestic supplies of cheap natural gas seeming to balloon, one would think that American motorists would be driving more natural gas cars nowadays. But that is far from the case.
Only about 150,000 natural gas-fueled vehicles travel on U.S. roads today, and most of those cars and trucks operate in centrally fueled commercial and municipal fleets. Meanwhile, some 15 million natural gas vehicles operate safely worldwide, mostly in Argentina, Brazil, India, Iran, and Pakistan.
Proponents in the U.S. say that it only makes sense to use inexpensive, abundant, and cleaner-burning methane gas as a fuel to power vehicles. Natural gas prices have fallen from 30 to 40%, and costs are somewhere around $1.50 less per gasoline-gallon equivalent. And when engines burn compressed natural gas (CNG) rather than gasoline, greenhouse gas emissions drop 28%, according to the California Air Resources Board. It also releases fewer smog-producing pollutants. Natural gas vehicles can meet California’s AT-PZEV emissions standard.
Natural gas cars and trucks are widely available in the U.S. Honda now offers the Civic Natural Gas (formerly GX) sedan in 36 states, and Chrysler is building a CNG pickup—the Ram 2500 4X4 Longbed. But conversion adds about $5650 to the price tag of the car and double that to the truck’s. Plus, CNG tanks take up cargo space and add weight, not to mention limit driving range. (Ford, Chrysler, and General Motors offer aftermarket CNG and/or CNG bi-fuel retrofits.)
Jim Lentz, President and CEO of Toyota USA, addressing the recent Los Angeles Auto Show, said that the looming prospect of "peak oil" means “there is going to be a point in time where natural gas is going to be one of the solutions for probably 25 to 30 years after that as we develop even more and more hydrogen…The question becomes, if an infrastructure is going to be built, will the investment be made in natural gas or in hydrogen.”
Future-tech planners at the U.S. Advanced Research Projects Agency-Energy (ARPA-e) have decided to lend CNG a hand. Last summer ARPA-e announced that it would present awards totaling $30 million to 13 research industry/academic projects that focus on expanding the use of natural gas as a vehicle fuel. The new R&D program, Methane Opportunities for Vehicular Energy (MOVE), aims to engineer lightweight, affordable natural gas tanks for cars and develop natural gas compressors that can efficiently refuel a natural gas vehicle at home, said Dane Boysen, Program Director at ARPA-e.
“Buying and running a natural gas vehicle is still too costly, and there’s no refueling infrastructure in the U.S. It’s only recently that the economic drivers dropped into place that justify adoption of natural gas fuel in light-duty vehicles,” he said.
Next-generation fuel tanks
Tank weight is a major issue, according to Boysen. Lightweight carbon-fiber pressure vessels are made using expensive filament-winding methods, whereas thick-walled metal tanks are heavy and costly. The tanks typically contain methane at pressures of 3000 or 3600 psi (207 or 249 bar). The ideal vehicular natural gas tank would be small, lightweight, essentially lossless, low-priced, and capable of meeting regulation automotive safety standards.
Boysen singled out a fuel-tank project at REL Inc., a custom equipment maker in Calumet, MI, to develop a low-cost, conformable natural gas tank for light-duty vehicles. REL received a $3 million award.
“Cylindrical pipe shapes don’t readily fit in vehicles,” said company Vice President Adam Loukus. “The question is: can you make something that’s lightweight, with no thick walls, that fits?”
Loukus thinks that an internal foam core design (perhaps using porous ceramic sponge partially infiltrated with metal alloys) may enable the tank to conform and achieve higher storage capacity at a significantly reduced cost. “In addition to increasing our energy density, this technology could also improve energy efficiency because it’s lightweight,” he said.
REL is also considering using “something like Lego blocks”—small linked cubes with lots of room inside to produce a continuous passageway for storage but also that provides mechanical strength. “Pressure wants to make everything spherical,” Loukus observed. The connected units could hold up against high pressure by tying mechanically into the opposite side of the tank.
Among other ARPA-e projects:
• Ford is engineering a tank that uses an innovative external framework and internal porous adsorption materials.
• The Gas Technology Institute is to identify new porous storage materials such as metal-organic frameworks, using a new computational screening tool. It’s also working on a thin pressure vessel shell with "nano-valves" that open and close on demand.
• OtherLab Inc. of San Francisco is to produce a low-cost tank that resembles a human intestine—filled with many small tubes.
• Pacific Northwest National Laboratory is incorporating high-strength internal strut technology. The tank is produced from aluminum-titanium alloy via superplastic forming and friction-stir welding.
• United Technologies is using modular designs and low-cost construction materials, allowing tanks to be manufactured into shapes that easily fit into the tight spaces of light-duty vehicles.
• SRI International is developing a container-less tank that uses strong, porous (high-surface area) carbon materials.
• Texas A&M University's technology involves highly adsorbent, low-cost materials for low-pressure storage tanks. “It is imperative to discover new porous materials that can effectively store and deliver methane at more reasonable pressures and temperatures,” said chemist Joe Zhou, the lead investigator.
Cheap home refueling
“There’s something fewer than a thousand natural gas stations in the nation,” said ARPA-e's Boysen. But with 60 million homes in the U.S. that have natural gas, “we already have the natural gas supply and distribution infrastructure; we just don’t have a refueling infrastructure.”
Low-cost home refueling pumps would help solve the problem.
“Honda had helped introduce a home compressor, but it cost $4000 or more, which is pretty much a nonstarter,” Boysen said. When connected to a natural gas line, Honda's "Phill" device can top off a vehicle overnight.
“We set the program goal of an entire home system costing under $1000, with $500 for the compressor,” said Boysen. “It brought out some interesting new technology proposals.”
Eaton, for example, is receiving $3.4 million to develop a home system that will use a liquid—acting like a piston—to compress natural gas.
“If you look at high cost of home refueling compressors,” said Clark Fortune, Principal Engineer at Eaton Innovation Center in Southfield, MI, “you see that if you have to draw too much energy out of the wall plug, you’re going to have to rewire the garage with high-power lines, which is [not a viable solution]. That…reduces the whole value proposition. The system needs to be efficient, low-cost and as inexpensive to use as possible.”
Eaton's idea is to “pump liquid into a pressure vessel to displace the gas inside. As long as the valve is closed, the gas will compress,” Fortune said. Two containers will exchange the liquid piston during gas pressurization via a cooled valve to reduce the temperature of the pressure-heated fluids.
“The goal is a $500 price tag—a cost reduction of nine to 10 times—with double the flow,” Fortune said.
MOVE will fund several other home compressor R&D projects:
• The University of Texas at Austin will develop a single-piston, four-stage compressor that’s driven by a linear motor—an integrated system that needs fewer moving parts.
• General Electric is to build a system that will chill, densify, and transfer natural gas more efficiently. The design has few moving parts, should operate quietly, and could be virtually maintenance-free.
• At Colorado State University, work focuses on using the vehicle engine itself to compress natural gas.